Adjustable power supplies (which are sometimes referred to as adaptable power supplies) are arranged to allow their output voltage to be varied so that it can be set at a required potential. There are several different techniques that can be employed to allow the output voltage to be varied. One of these techniques essentially involves obtaining a portion of the output voltage via a voltage divider network, and providing the portion of the output voltage to an electronic control circuit that is arranged to alter (if necessary) the output voltage such that the portion of the output voltage is substantially equivalent to a fixed internal reference voltage of the power supply. Thus, by changing a resistance of a particular ‘control’ resistor in the voltage divider network it is possible to vary the output voltage of the power supply. Changing the resistance of the control resistor will bring about a change in the potential of the portion of the output voltage, which in turn causes the power supply to alter its output voltage such that the portion of the output voltage is substantially equivalent to the internal reference voltage.
An example of where the above technique is used is in the X2 multi-source agreement. The X2 agreement defines a fiber-optical module that conforms to the 10 Gigabit Ethernet standard as specified in IEEE 802.3ae and which is to interface to an external adaptable power supply. The X2 agreement defines several discrete resistance values for the control resistor (in the aforementioned voltage divider network) and the associated output voltage that the adaptable power supply should produce for each of the discrete resistance values. The fiber-optical module is basically arranged to set the resistance of the control resistor to the discrete resistance values so as to set the output voltage of the adaptable power supply as required.
The developers of the X2 agreement selected the discrete resistance values for the control resistor based on the assumption that the adaptable power supply has an internal reference voltage of 0.8 volts. As such the discrete resistance values are such that there will be 0.8 volts across the control resistor (that is, the portion of the output voltage) for the associated output voltage of the adaptable power supply.
An undesirable consequence of having selected the discrete resistance values based on the assumption that adaptable power supplies use a 0.8 volt internal reference is that it restricts designers of adaptable power supplies to using only DC-DC converters that have a 0.8 volt reference, even though DC-DC converters are available in a range of reference voltages (which is typically 0.5 volts to 1.5 volts). Whilst designers can readily design adaptable power supplies using DC-DC converters that use a voltage reference other than 0.8 volts, such adaptable power supplies would not accord with the X2 multi-source agreement because the discrete resistance values of the control resistor would not result in the adaptable power supply producing the associated output voltage. For instance, instead of producing an output voltage of 1 volt for a control resistor of 3160 ohms (as specified in the X2 agreement) the adaptable power supply could output, for example, 1.6 volts.
Being restricted to using only DC-DC converters that use a 0.8 volt reference means that designers are not able to benefit from the various technical and economic benefits that could stem from being able to use DC-DC converters over a range of reference voltages.